Electrical follow-up motor control system



July 8, 1947.

J. w. DAwsoN 2,423,438

ELECTRICAL FOLLOW UP MOTOR CONTROL SYSTEM 3 Sheets-Sheet 2 Filed NOV. 9, 1943 i mi* INVENTOR.

JoH/v W. Dn wsa/v,

MRW

July 8, 1947. J. w. DAwsoN 4 R CONTROL SYSTEM 3 Sheets-Sheet 3 ELECTRICAL FOLLOW UP MOTO Filed Nov. 9, 1943 Aurunca 7, E, Y [cw INVENTOR. Jon/v #f Dnmso/v,

d trolled object Il, such as a Patented July 8, 1947 s PATENT ori-ICE ELECTRICAL FOLLOW-UP MOTOR CONTROL SYSTEM John W. Dawson,

West Newton, Mass.. assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application November 9,

s claims. (o1. 172-239) This invention relates to a follow-up system in which a controlled element, such as a directional radiating antenna structure, is caused to follow the rotation of a remotely-located controlling member. In such devices a tendency exists for the controlled element to overshoot its final or zero position as determined by the controlling member, and thus subject the system to a hunting action which introduces inaccuracies and delays in the operation of such systems.

An object of this invention is to devise a simple apparatus for substantially eliminating such overshooting or hunting tendencies in systems of the foregoing type.

Another object is-to devise a system in which y any tendency for the controlled element to lag behind the controlling element to an excessive degree is substantially eliminated.

. Another object is to devise tems of the foregoing type in VL ch the dynamic response thereof is improved,

A still further object is to accomplish the foregoing by an electrical control system which is simple and reliable.

The foregoing and other objects of this invention will be best understood from the following description of an exempllflcation thereof, reference being had to the accompanying drawing, wherein:

Fig. 1 is a schematic diagram of a system constructed in accordance with my invention;

Fig. 2 is a set of curves illustrating certain details of operation of the system shown in Fig, 1;

Fig. 3 is a schematic diagram of another embodiment of my invention; and f Fig. 4 is a vector diagram illustrating certain principles of operation of the system shown in Fig. 3.

In the system illustrated in Figs. 1 and 2, a condirectional radiating antenna system for radiating super-high frequency waves is to be controlled in its position by a remotely-located controlling member 2, such as a hand crank. Of course it is to be understood that this controlling member may take any other form which is well known in follow-up systems of this type. The controlled object I is driven to its desired position by means of a direct current commutator motor 3 having a field winding 4. The field winding is adapted to be supplied with magnetizing current from a suitable source of direct current, not shown, in order to energize the field of themotor. Motor 3 drives a drive shaft 5 preferably through a gear reduction drive 6. Controlled object I is mounted upon and moved by the drive shaft 5.

The motion of the controlling object 2 controls a synchro 1 consisting of a stator having a closed stator winding 8 and a movable armature 9 carrying a winding I0. Motion of the controlling 'ti-hunting sys- 1943, Serial N0. 509,631

object 2 will rotate the armature 9 in a clockwise or counterclockwise direction depending upon the direction of motion of said controlling object. Associated with the synchro 1 is a similar synchro II consisting of a stator having a closed winding I2 and a rotatable amature I3 carrying a winding I4. The armature I3 is likewise mounted upon and driven by the drive shaft 5. Three equally spaced points on the stator winding 8 are connected by conductors I5, Iii and I1 to three similarly spaced points on the stator winding I2. The armature winding I0 is energized with alternating current from a pair of terminals I8 which may be energized from a standard Bil-cycle, 11S-volt source.

As is well known, the alternating current field of the armature 9 is reproduced in the space within the stator winding I2, and induces a voltage in the armature winding I4-unless the armature I3 is substantially at right angles to said field. Thus the voltage induced in the winding I4 is a measure of the deviation of the armature I3, and likewise of the controlled member I from the zero position which is the desired position of coincidence between controlling member .2 and controlled member I.

The voltage generated in the armature winding I4 is conducted through an anti-hunt network I9, which will be described in detail below, to the primary winding 20 of anzinput transformer 2| which The ontrols a servo amplifier. servo amplifier cjmprises a pair of high vacuum triodes 22 and 23,

which in turn control a pair of thyratrons 24 and 25. The thyratrons 24 and 25 control the magnitude and direction of the current supplied to the /armature of motor 3, and thus control the magnitude and direction of the torque exerted by said motor.

The input transformer 2I is provided with a pair of secondary windings 26 and 21. The voltage of the secondary winding 26 is impressed between the control grid 28 of triode 22 and the cathode 29 of said triode. inserted in the lead 56 extending to the grid 28. The cathode lead 61 is connected to a tap 58 on a potentiometer 59 connected across a secondary winding 31 of a power supply transformer 32. In this way an adjustable alternating current bias is impressed on the grid 28 in addition to the voltage delivered by the secondary winding 20. The cathode 29may be of the indirectlyheated thermionic type provided with a suitable lamentary heater, not shown. The triode 22 is likewise provided with an anode 30 which `is connected through a secondary winding 3| of the power supply transformer 32.`and a resistance 33 to the cathode 29 of triode 22. The power supply transformer 32 is provided with a primary winding 34 which is likewise energized from the power supply terminals I8. Thus during A grid resistor is V l ,.witha plurality. oftaps' 48,

respect to the cathode 29, and the magnitude of l said pulse will be dependent upon the magntude of the voltage delivered to said grid28 by the secondary winding 26. The phase of the A, C, bias at the tap 58 is inopposition. to the v phase of the voltage on the anode 3l).A when the voltage delivered by the secondary winding 26 is in phase with the voltage-on the anode 30, the A. C. bias will tend to flatten Ithe voltage curve supplied to the grid 28. This accomplishes abeneilcial result which will be pointed out below.

Thevoltage developed across the resistance `33 is used to control the voltage on the control grid 35 of the thyratron 24, and thus to control the time at which thyratron 24 res. For this purpose the cathode 29 of triode 22 is connectedfthrough a network 36, which supplies a phase-shifted voltage, to the grid 35. Network 36 consists of the auxiliary secondary winding 31 on the transformer 32 with a condenser 38' and a resistance 38 connected in series across said secondary winding 31. The point between the resistance 39 and the condenser 38 is connected to the grid 35, while the other end of the resistance 39 isconnected to theY cathode 29. A condenser 4'l preferably is connected between the grid 35 and ,the4 cathode 42 of the thyratron 24. The cathode 42 may be of the lamentary 'thermionic type. y The thyratron 24 is'also provided with yan anode 43 which is connected .through asecondary Winding, on transformer v32, conductor 45, thearmature of motor'3, and

conductoriwbacktothe cathode42. ,A

The .same connections described/above'jior tubes 22 and 24 are likewise providedM fortubes" 23 and- .25 andthus in Fig 1. wh'erethecon- -nections and e1ements' are identical;v the" same `reference numbers, withthe addition of the letter a, areapplied thereto. It will benoted that the connections are so 'arranged that the 'current v delivered by thyrat'ron 24 flows through the armature of motor 3, from right to left, while .the current i delivered by thyratron 25 ilows `through thel armature vof motor 3, from left to right. Thus conduction through tube 24 will tend to lcause rotation of motor 3 in one direction, and conduction through tube 25 will tend tocause rotation of motor 3..in theopposlte direciion.`

'I'neanti-.huntiinetfwork H in'cludesanflauto-f transformer 41, the output olfy armature '|,4be'ing connected across) a `portion ofl the windings thereof. .Theauto-trensformer41 is provided I so that a'predeter-r mined ratio between'the input'and the output voltages ofsaid autoftransformer may be selected. Preferably acrossthe same portion of the auto-transformer which is connected to the output of armature winding I4 is connected an inductance 49 in parallei'vwith'a condenser 50,' said i parallel circultbeing'connected in series with an impedance 5i Ywhich is preferably a resistance.

- VThe primarywinding is connected between a contact52 on the 4selected tap '48 and 1avariable tap 5,3'on the resistance 5i. The parallel circuit comprising the inducftance 49 and thecondenser y50 lis.,tuned.tothe frequency,ofjthelvoltagesouree at theftermin'als '|8..

When the controlling'V-mernber 2 ismovedso as to produce an angular displacement of the armature 9, a voltage appears across the armature the iin'e voltage at the winding I4, `asf.alreadyidescribed. This voltage will be-eith'er'in phase 4or 180 out of phase with input terminals I8, de-

pending upon either the direction of rotation of the armatureg or upon whether the armature XI3 is leading or lagging 4the armature 9. This voltage, ,which is affected in magnitude by the y, anti-hunt circuit I8, as will be described below.

are substantiallyino displacements introducedl by the circuits themselves, so that said'anode voltages likewise can be'considered in phaseand 180 out of phaseA with the line voltage at `the terminals l8r The Yconnections between the'secondary windings 26 and 21 andthe respective grids 28 and 28a are so arrangedthat the voltages on said grids 28 and 28a`are in phase with each other. 'Thus when grid "28 `is positive with respect to its cathode'29, thegrid 28a will likewise be positive with respect to its cathode 28a.

The operation, particularly of the thyratrons 24 and 25, will be explained in connection with the curves as shown in Fig. 2. Along axis A are .plotted thel lvoltage conditions vof one of the thy'ratron's, for example ythyratron 25. ``The curve Ep represents a positive half cycle of volt.age as applied to the anode 43a.l rThe `curve C vis the'grid v characteristicl urve of tube 25;` representing values of grid volt el above' whicHtube'N will start conducting current,y The curve Eg represents the voltage which 'thefnetwoi-k 36'afimpresse'sr on the grid 35a. It will ybe seen that the' voltagefEd isso delayed in phase rwith respect to tbevoltage Ep,

'that Eg does `not intersectC until at 'a time T4,

r,which is very late inl thepositive'half 1 cycle. v`-With voltage l'llg` aloner impresseduponithje Igrid 35a, tube 25 will conduct substantially no current." We

ymay now assume that the armature isjdis'placed A of phase with the anode voltage so as to be negative with respect to tlf1e"'cathodev `29a' throughout the time thatjtheanode `4'prrevents saidy tube' 23 jfrom 30a isposltive. 5 This conducting current,

l and thus rio-"voltagewilli appear acrossthe re- 'sistance 33a.

Under `these conditions the only voltage whichis impressed upon the grid 35a is that of the network 36a,` so that the tube 25 conducts substantially no current asdescribed above in connection with Fig. 2. i

As the anode 30 of tube 22 becomes positive, the grid 28 will have arpositive voltage impressed vupon it, which voltage will be governed in magnitude in the irst instanec by the, magnitude of the voltage generated by the armature winding i4. The armature voltage is in turnproportional to the angular vdeviation between armatures Sand i3. The anti-hunt circuit i9. may introduce an additional variation in the magnitude of the voltage impressedon\thegrid28. aS, will be described morez'fully below.` The positive voltage thus impressed on grid 28 causes-a flow',V of current through the tube 22. This ilow of current produces a voltage drop through resistance 33.

In Fig. 2, along axis B, are plotted a number of position as determined by the setting or motion of different magnitudes of voltage Ear, Eaz, Eos, as the controlling member 2. developed across the resistance 33. The curves In practice the variation in the voltage of the as plotted along axis A of course apply equally grids 28 and 28a in phase with their respective to tube 24. It will be seen that in the circuit o o anodes 30 and 30a tends to impart to the voltage Fig. 1, the voltage developed across the resistance developed across the resistances 33 and 33a a 33 is added vectorially to the voltage across the peaked third harmonic effect. The inclusion network 36. 'Along axis A the results of such of the A. C. bias at the taps 5B and 58a tends veewriai additions between the voltage Eg and to counteract this tendency by ettenlng outth: the voltages Ear, Eaz and Ea: are plotted as Eri, grid voltage curve as described above. Under these conditions the sine wave curves of Fig. 2B

En and En. It will be seen that as the magare more closely approximated.

nitude of the voltage Ea increases, the voltage Er intersects the grid characteristics much earlier In abSenCe 0f the anti-hunt circuit I9, the conin tbe voltage eyeie Ep, so that tube 24 wiii be trelled member l under the foregoing System ignited at the times Tr, T2 and T3, with the Cor- 15 might tend to overshoot or hunt. This tendency,

responding voltages Ear, Ea: and Ea3 Thus an however, is substantially eliminated by the ce of this anti-hunt circuit. If an angular increase in the angular deviation between the presen deviation is produced between the armatures armatures 9 and i3 will advance the ring time of the tube 24 and cause it to denver an increased 9 and t3, a voltage will be generated in the armacurr-ent flow to the armature of motor 3. This 2o ture winding I4, as already described. 1f this current ows through armature from angular deviation does not Change, the amplitude to left, and produces energization of the motor ,0f the Voltage Supplied by the armature Winding ond rotation of its armature Thereupon tho I4 will likewise remain fixed. Since the parallel motor 3 tends to rotate in (one direction to drive Circuit Consisting 0f the inductance 9 and con' denser 59 is tuned to the frequency of this voltage,

the armature i3 toward its zero position. 25

Upon reversal of the phase or the voltage at it will act as a substantially innite impedance the input terminals I8, anodes and 43 become to salu Voltage which is unpressed upon u' I negative, and the tubes 22 and 24 cease conductau actual embodiment' or course' the uupeuance is not infinite. However, the higher the Q of At the same time the voltage on circuit 49, 50 is made, the more nearly will said ing current.

positive. How- 30 circuit act as an infinite impedance. As is well the anodes 30a and 43a becomes ever, the voltage across the secondary winding 2 has reversed causing lthe grid 28a to become negknown ative with respect to its cathode 29a. Thus the oL tube 23 and its eontreiied tube z5 de not een- 35 Q"R i dutncexsscgf rterxrshs Volt; where 3:2102 and L is the iriductance of element 49, R is the resistance of the parallel circuit age, the anodes 30 and 43 of tubes 22 and 24 again 49 50, and f is the frequency of the voltage Sup become positive, and the action previously deplied thereto Values of Q of at, least 3o are With scribed is repeated, resulting in the supply readily obtainable 1n a practical system.

through the armature of motor 3 of another pulse Such values for Q the parallel circuit o 5o Wm.

impedance to a conof direct current flowing from right to left present a sumcientry high impressed thereon so through said armature. As long as the armature Stam am plitude of voltage I3 has not been dmven to its Zero posmon by the as to limit the iiow of current through the relly negligible value.

motor, the above action is repeated each half t 5 b t. cycle, and the armature of motor 3 continues to 45 lrthe' atov nsi of a axe be Supplied with rectmeu direct current flowing deviation between the armatures 9 and from right to left through Said armature' .Said magnitude of the control voltages supplied to rectified current consisting of pulses of direct the grids 28 and 28a Wm be governed solely by current Supphed from the tube When the said angular deviation. If, however, said angular armature I3 reaches its zero position, the voltdeviation decreases at a given rate, the voltage age across the Secondary Wlpdlug T6 drops to Zero across the parallel circuit 49, 50 will likewise and the tube n together wuh us Controlled tube decrease at said given rate. Undery these condicease to denver an appremable amount of cur' tions the 'circuit 43, 50 tends to operate as an rent' whereupon the armature of motor 3 is no 55 oscillator, and attempts to maintain the voltage longer Supplied Wu'h sufficient current to produce across the inductance 49 at the previously estab- Therefore, an unbalance between rotation, and said motor stops. Shed lover If We assume now that the coudmous of m0' the voltage which appears across the lower porto generate i tion of the auto-transformer 41, and the voltage tion of the armature 9 are such as l a voltage in the armature winding i4 which 1s 60 which appears across the nductance o wm be in a ilow of current through 180 out of phase with the line voltage, the con- Created, resulting UOllilg VOltage appearing across the. Seconda@ the resistance 5 l. This voltage will be subtracted 1 gud 28u" Wm from the voltage across the upper portion of the winding 21, as impressed upon 0W be in phase With the Voltage on the anode auto-transformer 4l which is impressed on the The net effect, therefore, of such 39a, while ythe voltage impressed on the grid 2B 65 winding M will be out of phase with the voltage on its anode o rato of decrease of the angular deviation be- 30a. Under these conditiOIlSy tubes 23 and 25 Wm tween the armatures 9 and i3 will be a reduction t OWng in the magnitude oi the voltage supplied to theoperate to supply pulses of direct curren nre 0f mOtOr 3v from left t0 primary winding 20.` This in turn will produce a through the armat n the amount of current as supplied right, in a manner similar to that described above decrease i On of tubes 22 to the armature oi motor 3 in accordance with in connection with the operati and 24. Under these conditions the armature of the description as given above. yAs the rate o! motor 3 will rOtate in the opposite dIGCtOn SO decrease of the angular deviation between the armatures 9 and i3 increases, the voltage appear-` mature l3 to its zero position,

l moves to the ing across the resistance 5i will increase, resultas to drive the ar in which the controlled member ing in a greater reduction in the magnitude of the control voltages as applied to the grids 28 and 28a.

It will be recognized that if the armature 9 is rotated at a high angular speed and the armature I3 rotates at a correspondingly high angular speed, no anti-hunt influence is required. It is only when the angular deviation between the armatures 9 and I3 starts to decrease at a substantial rate that some anti-hunt influence should be exerted upon the system. From the above description it will be seen that the anti-hunt operation of the circuit I9 follows the above desired relationship.

As the angular deviation between the armatures 9 and I3 decreases, the voltage across the upper portion of auto-transformer 41 supplied to the winding 20 will decrease. Therefore, if said angular deviation decreases at a sufliciently rapid rate, it is possible for the voltage developed across the resistance 5I at the tap 53 to exceed the voltage across the upper portion of the autotransformer 41, and thus reverse the phase of the control voltage as applied to the primary winding 20. This will not only decrease the magnitude of the current as supplied to the armature of motor 3, but will actually extinguish the previously conducting thyratron and cause the previously non-conducting thyratron to conduct current. This will produce a reversal in the voltage applied to the armature of motor 3, and exert a dynamic braking action on said armature and thus on the motion of the controlled object I. Thus it will be seen that such a dynamic braking action will be exerted in cases where otherwise an overshooting of the nal position of the controlled object I might occur.

The network I9 not only operates so as to overcome tendencies for the system to hunt, but also acts to speed up the operation of the system 'where the controlled object I tends to move toward the iinal predetermined position too slowly. If the angular deviation between the armatures 9 and I3 starts to increase at a given rate, it will be seen that the voltage across the lower portion of the auto-transformer 41 will exceed the voltage established across the inductance 49, and thus produce a flow of current through the resistance 5I, which is in the opposite vectorial direction to that which occurs when a decrease in the angular deviation takes place. Under conditions of an increasing angular deviation, the voltage across the resistance 5I delivered to the tap 53 will be added to the voltage supplied from the upper portion of the autotransformer 41, and thus increase the current flow as delivered by the conducting thyratron This will impart an added-accelerating force to the armature 3 so as to tend to decrease the angular deviation between the armatures 9 and i3.

The degree to which the parallel circuit 49, 50 imparts the foregoing anti-hunt and accelerating influences can be adjusted by adjusting the positions of the taps 52 and 53.

The system described can be analyzed on a basis of the vectorial factors of the lsystem in terms of its hunting frequency. In any followup system, the system as a whole, when possessing hunting tendencies, is closely analogous to an oscillating electrical system. The inertia 0f such a system can be represented as an inductance L. the friction or damping of the system can be represented as a resistance R, and the motor together with its restoring force tending l f :MLC

If the frequency of E is also equal to f, then the system will oscillate with the mass swinging back and forth past the given position without ever stopping at such position as desired. The reason why such a system oscillates is that E is usually delayed in phase with respect to the oscillation of the mass so that as said mass swings through the desired or zero position, the voltage E still tends to drive the motor in the same direction past said zero position, thus feeding energy into the system in such a direction as to sustain or even increase the oscillations. sidered as the signal voltage varying in magnitude at the hunting frequency (not the 60-cycle control frequency), Normally any such system introduces lags in the signal voltage as expressed in terms of E, and thus any such system tends to have a permanent hunting characteristic. If, however, E were to be advanced in phase so as to be in phase or leading with respect to the swinging of the mass through its zero position, then the oscillations would be damped quickly. By proper selection of the system constants, the syst-em will be in a non-oscillatory condition in which there is no tendency for the mass t0 swing beyond its zero position, In the system as described in Figs. 1 and 2, it will be noted that the anti-hunt circuit I9 produces a phase shift in E which advances E with respect to the oscillations oi the mass of the motor 3 and its associated load to such an extent as to substantially eliminate hunting tendencies.

Since, as pointed out above, the hunting variations of a follow-up system occur in alternating form at a hunting frequency, the foregoing analysis can be applied to anti-hunt systems in which a D. C. control voltage is utilized instead of the (iO-cycle A. C. control voltage as described in connection with Fig. l. Such an alternative arrangement is shown in Fig. 3, in which the same reference numerals are applied where the elements are identical with those of Fig. l. In Fig. 3, however, the controlling object 2 regulates a bridge potentiometer 60 consisting of a resistance 5I connected in a closed loop and a movable arm 62 carrying brushes 63 and 64, which contact the resistance 6I at diametrically opposite points. Motion of the controlling object 2 will rotate the arm 62. A suitable source of direct current 65 is connected to the brushes 63 and 64. Three equally-spaced points on the resistance 6I are connected by conductors 66, 61 and 68 to three similarly spaced points on the closed resistor 69 of a similar bridge potentiometer 10. The bridge potentiometer 10 likewise has a rotatable arm 1I carrying brushes 12 and 13 contacting the resistance 69 at diametrioally opposite points. The arm 1I is mounted upon and driven by the shaft 5. The brushes 12 and 13 are connected by conductors 14 and 15 to the opposite ends of a potentiometer 16. A condenser 11 is connected from one end of the potentiometer 16 through a resistance 18 to an intermediate tap 19 on said potentiometer 16. A tap on the resistance 18 is connected to a conductor 8|, while the lower end of the potentiometer 16 is connected to a conductor 82. The conductors II and I2 extend to an amplifier 83, the

E can be con-' tor, provided such output of which is supplied by means of conductors 84 to the armature of motor 3.

As the controlling object signal voltage will be produced at the brushes 12 and 13, which will produce an operation of the amplifier 83 to operate the motor so as to drive the controlled object l toward a position of agreement with the controlling object 2 in a manner similar to that described above in connection with Fig. l.

In Fig. 3 the control voltage instead of being a 60-cycle alternating voltage is a substantially unidirectional voltage. As pointed out above, such a system may possess hunting tendencies which can be expressed in terms of alternating values at the natural hunting frequencies of the system. In Fig. 4 the vector relationships of the system shown in Fig. 3 are illustrated, said vectors being in terms of the natural hunting frequency of the system. The signal voltage delivered by the conductors 14 and 15 is represented as Es. The condenser 11 would tend to have a voltage developed across it, which in terms of the hunting frequency of the system lags the voltage Es. This lagging voltage is represented in Fig. 4 as XcI. The voltage developed across the resistance 18 will, of course, be at right angles to the voltage XcI, and is represented as RI. The vector sum of XcI and RI is that portion of the voltage E, which occurs at T1 corresponding to the position of the tap 19. The point Tz and Ri corresponds to the position of the tap 80 on resistance 18. The control voltage which occurs across the conductors 8| and 82 will thus be the voltage Ec extending from Tn to the end of En. It will be noted that the voltage Ee leads the voltage Es by a substantial amount. The angle of lead can be so selected as to overcome the angular delay introduced into the system, and thus substantially eliminate hunting tendencies.

Of course, it is to be understood that this invention is not limited to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. For example, widely differing types of amplifying systems could be devised to supply current to the moamplifying systems would respond to the magnitude of the control voltage as delivered by the armature winding i4 and the associated anti-hunt circuit I9. Thus, for example, instead of the triodes 22 and 23, pentodes with suitable biases on the grids might be utilized.

What is claimed is:

1. In a follow-up system, a controlling member adapted to be moved from one position to another, a controlled member adapted to be moved correspondingly, means for generating a voltage of a predetermined frequency in response to a deviation between the positions of said controlled and controlling members, means for connecting said voltage in series with an impedance means across a circuit consisting of an inductor and a capacitor connected in parallel and tuned to said predetermined frequency, means for deriving a control voltage equal to the difference between a voltage proportional to said first-named voltage and the voltage across said impedance means, driving means for driving said controlled member toward a position f agreement with said controlling member, and means for controlling said driving means in accordance with the magnitude of said control voltage.

2. In a follow-up system, a controlling member adapted to be moved from one position to another, a controlled member adapted to be moved corre- 2 is moved, a D. C.

spondingly, means for generating a voltage of a predetermined frequency in response to a deviation between the positions of said controlled and controlling members, means for connecting said voltage in seri-s with a resistor across a circuitL consisting of an inductor and a capacitor connected in parallel and tuned to said predetermined frequency, means for deriving a control voltage equal to the difference between a voltage proportional to said first-named voltage and the voltage across said resistor, driving means for driving said controlled member toward a position of agreement with said controlling member, and means for controlling said driving means in accordance with the magnitude of said control voltage.

3. In a follow-up system, a controlling member adapted to be moved from one position to another, a controlled member adapted to be moved correspondingly, means for generating a voltage of a predetermined frequency in response to a deviation between the positions of said controlled and controlling members, said voltage being reversible in phase depending upon the direction of rotation of said controlling member and upon whether said controlled member is leading or lagging said controlling member, means for connecting said voltage in series with an impedance means across a circuit consisting of an inductor and a capacitor connected in parallel and tuned to said predetermined frequency, means for deriving a control voltage equal to the difference between a voltage proportional to said first-named voltage and the voltage across said impedance means, reversible driving means for driving said controlled member toward a position of agreement with said controlling member, and means for controlling the force of said driving means in accordance with the magnitude of said control voltage and for controlling the direction of the application of the force of said driving means in accordance with the phase of said control voltage.

4. In a follow-up system, a controlling member adapted to be moved from oneposition to another, a controlled member. adapted to be moved correspondingly, means for generating a voltage of a predetermined frequency in response to a deviation between the positions of said controlled and controlling members, said voltage being reversible in phase depending upon the direction of rotation of said controlling member and upon whether said controlled member is leading or lagging said controlling member, means for connecting said voltage in series with a resistor across a circuit consisting of an inductor and a capacitor connected in parallel and tuned to said predetermined frequency, means for deriving a control voltage equal to the difference between a voltage proportional to said first-named voltage and the voltage across said resistor, reversible driving means for driving said controlled member toward a position of agreement with said controlling member, and means for controlling the force of said driving means in accordance with the magnitude of said control voltage and for controlling the direction of the application of the force of said driving means in accordance with the phase of said controlvoltage, Y

5. In a follow-up system, a controlling member adapted to be moved from one position to another, a controlled member adapted to be moved correspondingly, means for generating a voltage of a predetermined frequency in response to a deviation between the positions of said controlled and controlling members, means for connecting said voltage in series with an impedance means across u circuit consisting of an inductor and a capacitor connected in parallel and tuned to said predetermined frequency, means for deriving a control voltage equal to the diierence between a volta-ge proportional to said first-named voltage and the voltage across said impedance means, drivinCr means for driving said controlled member toward a position of agreement with said controlling member', a controlled ignition type tube for delivering energizing current to said drivin;r means, said tube normally having sucli a delayed tiring angle as to deliver substantially7 no current to said driving means, and means for advancing the tiring angle of said tube in accordance with the magnitude of said control voltage. 6. In a follow-up system, a controlling member adapted to be moved from one position to another, a controlled member adapted to be moved from one position to another, means for generating a voltage of a predetermined frequency in respense to a deviation between the positions of said controlled and controlling members, means for connecting said voltage in series with an impedance means across a circuit consisting of an inductor' and capacitor connected in parallel and tuned to said predetermined frequency, for deriving a control voltage equal to tne diriercnce between a voltage proportional to said first-named voltage and the voltage across said impedance means, reversible driving means for driving said controlled member toward a position of reement with said controlling member, said drV.y ng means comprising a direct current motor driving force reverses with a reversal of current supplied thereto, a pair of controlled itior; type rectifying tubes for delivering ene izing current to said driving means, said tubes operating to deliver current to said driving means in opposite directions, each of said tubes normally having such a delayed ring angle as to deliver substantially no current to said driving means, and means for causing one of said tubes to conduct when said control voltage has a predetermined phase and for causing the other of said tubes to conduct when said control voltage has the opposite phase and for advancing the firing 12 angle of the conducting tube in accordance with the magnitude of said control voltage.

7. In a follow-up system, a controlling member adapted to be moved from one position to another, a controlled member adapted to be moved correspondingly, means for generating an alternating control voltage of a predetermined frequency in response to a deviation between the positions of said controlled and controlling meznbers, driving means for driving said controlled member toward a position of agreement with said controlling member, means for controlling said driving means in accordance with the magnitude of said control voltage, and antihunt means comprising an inductor and a capacitor in parallel tuned to said predetermined frequency and connected to said control voltage, and means responsive to the response of said parallel inductor and capacitor circuit, for changing the response of said driving means to said control voltage.

8. In an electrical control system, means for producing an alternating control voltage of a predetermined frequency, controlled means responsive to said control voltage to produce an electrical output in accordance with said control voltage, said system having a substantial inertia tending to produce hunting, and anti-hunt means comprising an inductor and a capacitor in paralle] tuned to said predetermined frequency and connected to said control voltage, and means responsive to the response of said parallel inductor and capacior circuit for changing the response of said controlled means to said control voltage.

JOHN W. DAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,684,137 Mittag Sept. 11, 1928 2,088,654 Hull Aug. 3, 1937 2,233,415 Hull Mar. 4, 1941 1,998,938 Mittag 'Apn 23, 1935 1,554,698 Alexanderson Sept. 22, 1925 

